Unidirectional electret condenser microphones (ECM”s) typically include a housing, a diaphragm and a ring assembly, a backplate, and a spacer separating the diaphragm and ring assembly from the backplate. The ECM may also include an amplifier that may be disposed on a printed circuit board electrically coupled to the backplate. These components are mounted within the housing. One way in which ECMs operate is by allowing acoustic vibrations to enter the housing and allowing the diaphragm to vibrate in response thereto. The vibrating diaphragm causes a capacitance change between the diaphragm and the backplate that may be detected as an electrical signal. The electrical signal is coupled to the amplifier by a suitable conductor, such as wire, to produce an output from the ECM.
Typically, the unidirectional ECM should provide a high performance and control so that the sound coming from the front of the microphone is reinforced and the sound coming from the back is canceled. A unidirectional ECM may be made directional in order to enhance the performance with respect to sound coming from the front of the microphone by adding a second sound inlet port, such that there is one at the front and one at the back of the ECM. The sound entering from the front of the microphone goes directly to the diaphragm. The sound entering from the back of the microphone is delayed by a resistive/capacitive (RC) acoustic network. This delay is made so that the sound coming from the front of the microphone is reinforced and the sound from the back is cancelled.
To implement the RC acoustic network, an acoustic resistive material may be disposed between the second sound inlet port and the diaphragm. This material may be made of sintered plastics, plastic felts, laser drilled disks, and the sound is made to travel through the material perpendicular to a plane of the material. That is, the material is typically provided in the form of a sheet or layer having a first surface and a second surface. The sound is then made to travel substantially perpendicular to the first and second surfaces.
This arrangement of the acoustic resistive material has several disadvantages. The acoustic resistive materials often have a relatively large amount of variability that has a great effect on the directional performance of the microphone, with laser drilled disks providing the least amount of variability of the currently available materials but at higher cost. Also, the physical volume of the material places limits on the size of the ECM making size reductions difficult.
Accordingly, there is a need for an ECM that is inexpensive, simple to manufacture and scalable to relatively small sizes.